1. Field of the Invention
The present invention relates to a mobile radio-frequency identification (RFID) and more particularly to a mobile RFID reader and a method of eliminating a transmission leakage signal in the mobile RFID reader.
2. Description of the Related Art
Generally, a radio-frequency identification (RFID) reader is either a fixed type or a mobile type, so that the RFID reader provides power to an electronic tag and reads an RFID code that is provided from the electronic tag.
Frequency bands used by the RFID reader is classified into a 125 kHz/134 kHz band, a 13.56 MHz band, a 433 MHz band, a 900 MHz band, a 2.45 GHz band, and a 5.8 GHz band.
Nowadays, a research related to RFID technology in the 900 MHz band is being actively conducted, and the 900 MHz band is similar to a band used by mobile devices such as cell phones.
Presently, a 900 MHz reader is a fixed reader based on a patch-type antenna. Companies such as Alien and AWID manufacture internal antenna-based mobile readers. Influenced by Wal-Mart, a North American-type reader is also manufactured. The frequency band used by the North American-type reader is 902-928 MHz, and the North American-type reader uses the frequency-hopping spread spectrum (FHSS) method and conforms to Federal Communications Commission (FCC) standards.
The latest reader technology tends to follow demand in the physical distribution field, beginning with retailers such as Wal-Mart. Most of the readers have a mobile reader module. There are several types of mobile reader modules, including a type that is applied to an industrial personal digital assistant (PDA), a Personal Computer Memory Card International Association (PCMCIA) (now PC Card) type, and a module type, according to a type of reader. Readers may be classified according to power source, including built-in battery-type readers and removable battery-type readers. There is a difference in power consumption according to whether a 0.5 W module or a 1 W module is used. The latest technology allows a reader to have a continuous operation time of 1 to 2 hours. There is no reader that is yet able to satisfy user demands in terms of operation time and performance. A system-on-chip (SOC) of the RFID reader is in demand so that the RFID reader can be built into a cell phone, because recent cell phones tend to be small, lightweight and slim.
The RFID reader usually uses just one antenna, and a transmitter unit and a receiver unit are divided by a circulator because the RFID reader transmits and receives signals within the same frequency band of 910-940 MHz.
If a transmission power is 25 dBm (250 mW), and a circulator isolation is 25 dB, which is currently the maximum isolation for circulators, and the transmission power is leaked to a receiver unit, 0 dBm of power is inputted to a receiver unit. To solve the problem that is described above, the RFID reader may have a separate passive device (isolator or coupler) whose isolation is greater than 40 dB. However, the separate passive device has to be longer than 20 cm in size in order to satisfy the high isolation. Therefore, it is impossible to apply the separate passive device to an RFID reader that is built into a cell phone.